Abstract

The formation, viscous evolution, and gravitational stability of
protoplanetary disks are investigated. The formation process is
parameterized by the angular velocity of the molecular cloud core omega,
while the viscous evolution is parameterized by the viscosity parameter
alpha in the disk; in this study we consider a range of (0.4-6) x
10-14/s for omega and from 10-5 to 10-1
for alpha. The axisymmetric gravitational stabilities of the disks are
checked using Toomre's criterion. The resulting disk surface temperature
distribution, (d log Ts/d log R) approximately = -0.6 (R is
the cylindrical radius), can be attributed to two heating sources: the
viscous heating dominant in the inner disk region, and the accretion
shock heating dominant in the outer disk region. This surface
temperature distribution matches that observed in many disks around
young stellar objects. During the infall stage, disks with alpha less
than 10-1.5 become gravitationally unstable independent of
omega. The gravitational instabilities occur at radii ranging from 5 to
40 AU. The ratio of the disk mass to the central star mass ranges from
0.2 to 0.5 at the times of instability, about 4 x 10-5 x
(omega/10-14/s)-0.67 yr. Most disks with low alpha
and high omega become gravitationally unstable during their formation
phase.

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